Waste water treatment methods include a membrane bioreactor method of immersing a filtration module in an activated sludge tank and allowing the filtration module to separate activated sludge from treated water by means of filtration, that is, performing liquid-solid separation. This method allows the filtration treatment to be achieved so as to significantly increase the concentration of activated sludge (MLSS: Mixed Liquor Suspended Solid) from 5,000 mg/l to 20,000 mg/l. The method thus has the advantage of allowing the volume of the activated sludge tank to be reduced or enabling the time required for reaction in the activated sludge tank to be reduced. Since the filtration is performed using membranes, no suspended solids (SS) are mixed into the treated water, thus eliminating the need for a final sedimentation tank. This advantageously enables a reduction in the ground-floor area of a treatment facility. Furthermore, the filtration can be achieved regardless of how activated sludge settles out. This allows the burden of activated sludge management to be reduced. Because of the large number of advantages described above, the membrane bioreactor has recently been prevailing rapidly.
If hollow fiber membranes are used for the filtration module, the strength of the membranes themselves prevents the surface of each of the membranes from being damaged upon coming into contact with foreign matter from raw water. The filtration module can thus last for a long period. Such a filtration module further has the advantage of being capable of back washing in which a medium such as treated water is ejected in a direction opposite to the filtration direction to remove attachments from the membrane surface. However, if an aggregate of activated sludge accumulated in the gap between the hollow fiber membranes or the foreign matter from the raw water (hereinafter referred to as suspended solids) is not removed during the filtration, effective membrane area may decrease. This may disadvantageously reduce filtration efficiency, thus preventing stable filtration over a long period.
Aeration is conventionally performed from the bottom of the module in order to avoid accumulating suspended solids on the surfaces of the hollow fiber membranes or between the hollow fiber membranes. Thus, based on the vibration effect of the hollow fiber membranes and a stirring effect associated with upward movement of bubbles, the suspended solids are peeled off from the surfaces of the hollow fiber membranes or from between the hollow fiber membranes to prevent the accumulation. For example, a skirt is installed at the bottom of the hollow fiber membrane module, with a plurality of through-holes formed in a skirt-side fixing layer. Thus, aeration is performed from the bottom of the skirt to form a gas-liquid mixture layer in the end of the skirt projecting from the skirt. Thus, bubbles are generated evenly through the plurality of through-holes to allow the suspended solids accumulated on the outer surfaces of the hollow fiber membranes to be easily peeled off.
However, when activated sludge with a high MLSS is filtered as in the case of the membrane bioreactor, in a bundle of hollow fiber membranes located close to a head, the suspended solids entrained in a gas-liquid multiphase flow may fail to pass completely through the bundle of the hollow fiber membranes. Then, the suspended solids may disadvantageously accumulate gradually.
To solve these problems, a method has been proposed in which hollow fiber membranes are divided into a plurality of small bundles at the interface of an upper fixing portion so that the small bundles are arranged at a specific distance from one another (see WO 2004-112944).